Previously known from the applicant's earlier invention No. FI-891706 is a method of driving a drilling apparatus forward, wherein the drilling apparatus is forced from the working pit into the tunnel by means of protecting tubes, the driving power unit being in the working pit. The same method is also known from the publication GB-2 157 344.
U.S. Pat. No. 2,669,441 discloses how the drill head is driven forward by means of a protecting tube provided with an internal spiral system which steers excavated material towards the tunnel opening.
Driving the drill head forward by means of protecting tubes, which are to be installed in the tunnel, is applicable to drillings in normal and watery terrains, wherein a protection tube is often an absolute necessity because of landslippage. It is quite clear that driving the drilling apparatus forward by means of protecting tubes is also advantageous since protecting tubes must be forced into the tunnel anyway. On the other hand, a protection tube is not usually needed when drilling into rock, in which case this method is not applicable.
Using a rotating tube for driving an apparatus forward is also applicable for tunnel drilling, but in this case, tee protecting tube is fitted with rotating elements in the rocky wall and the other end of the protecting tube needs at least one great thrust bearing. The joints between the protection tubes, which are arranged sequentially, must be lap joints. In addition, in this case, the directional stability is not the best possible.